Optical recording/reproduction apparatus

Abstract: An optical recording/reproducing apparatus includes an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium having a plurality of neighboring tracks as a fine light spot so as to perform recording/reproduction of information or reproduction of information, and a detection optical system for detecting a returned light beam from the optical recording medium. A mask is arranged in a far field region sufficiently separated from a focal plane of the detection optical system, for masking marginal rays, in a direction perpendicular to the track, of the returned light beam, so that information reproduced from a track adjacent to the predetermined track upon reproduction of information on the predetermined track is reduced.

Description: The present invention relates to an optical recording/reproduction apparatus for recording/reproducing information on/from or reproducing information from an optical disc, a magneto-optical disc, or the like and, more particularly, to an optical arrangement which reduces crosstalk from an adjacent track and intersymbol interaction from an adjacent mark by inserting an aperture in the pupil or the far field region of a detection optical system for detecting a returned light beam from an optical disc so as to prevent marginal rays from becoming incident oh a photodetector. In recent years, various optical memories which perform recording/reproduction using a semiconductor laser beam have been placed on the market, and in particular, magneto-optical recording/reproduction apparatuses which can rewrite information are considered to be promising. A magneto-optical recording/reproduction apparatus magnetically records information by utilizing a local temperature rise of a magnetic thin film upon spot irradiation of a laser beam, and reproduces information by a magneto-optical effect (Kerr effect). Recently, as the information amount to be processed by, e.g., a computer has increased, a further increase in density of the magneto-optical recording/reproduction apparatus has been studied. In order to increase the density, the interval between two adjacent recording tracks is decreased, the length of each recorded mark is shortened, or mark edge recording is adopted in place of mark position recording. On the other hand, in an optical head as well, the wavelength of a laser beam is reduced to decrease the light spot diameter, or the super-resolution is used so as to increase the density. An optical system of an optical head using the super-resolution in a conventional optical recording/reproduction apparatus will be described below. An example of application of the super-resolution to a light spot on an optical disc using an annular aperture described in Japanese Laid-Open Patent Application No. 56-116004 will be described below with reference to Fig. 1. A light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and is then incident on a beam splitter 3. The light beam transmitted through the beam splitter 3 forms a fine light spot 9 on a magneto-optical disc 5 by an objective lens 4 via an annular aperture 8. The annular aperture is constituted by arranging a circular mask (diameter e) at the central portion of a normal circular aperture (diameter A). Returned light from the magneto-optical disc 5 is reflected by the beam splitter 3 via the objective lens 4, and is guided to a photodetector 7 by a condenser lens 6. Fig. 2 shows the sectional shape of the light spot 9 obtained when the annular aperture is used. In Fig. 2, the abscissa represents a value in units of NA/λ where λ is the wavelength of a light beam from the semiconductor laser 1, and NA is the numerical aperture of the objective lens 4. The ordinate represents a value normalized with the central intensity of the light spot 9. For the sake of simplicity, a case will be examined below wherein a light beam having an almost uniform intensity distribution is incident on the objective lens. A curve a corresponds to a circular aperture, and a curve b corresponds to the annular aperture having ε = 0.5A. If the light spot diameter is defined by an Airy disk, the light spot diameter obtained when an annular aperture is used decreases to about 82% with respect to that of a circular aperture. The resolution of the optical system can be improved accordingly. However, the intensity of a side lobe of the light spot is as low as 2% or less of the central intensity when a circular aperture is used, while the intensity of the side lobe is as high as 10% when an annular aperture is used. For this reason, the crosstalk amount from an adjacent track and the intersymbol interaction amount from an adjacent mark undesirably increase. In a magneto-optical disc apparatus which requires high power upon recording/erasing of information, light utilization efficiency is considerably lowered when an annular aperture is arranged. An example of application of the super-resolution to a light spot on a photodetector using a pinhole described in Japanese Laid-Open Patent Application No. 2-168439 will be described below with reference to Fig. 3. The same reference numerals in Fig. 3 denote optical parts having the same functions as in Fig. 1. A light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and is incident on a beam splitter 3. The light beam transmitted through the beam splitter 3 forms a fine light spot 9 on a magneto-optical disc 5 by an objective lens 4. Returned light from the magneto-optical disc 5 is reflected by the beam splitter 3 via the objective lens 4, and is guided to a photodetector 7 by a condenser lens 6. A pinhole 11 is arranged at the focal point position of the condenser lens 6 in front of the photodetector 7. Fig. 4 shows the shape of a light spot 10 at the focal point position in an enlarged scale. The light spot 10 has a similar shape to that shown in Fig. 2. The pinhole 11 allows only the central portion of the light spot to pass therethrough, and guides it to the photodetector 7, thus masking the side lobe. For this reason, crosstalk components from an adjacent track and intersymbol interaction components from an adjacent mark in the track direction included in the side lobe portion can be removed. For example, if the focal lengths of the objective lens and the condenser lens are respectively set to be fo = 3 mm and fc = 30 mm, the NA of the objective lens is set to be 0.55, and the wavelength of the semiconductor laser is set to be λ = 780 nm, the spot diameter (defined by 1/e 2 of the central intensity) of the light spot 10 is about 12 µm, and the diameter of the pinhole for masking the side lobe must be about 15 µm. Therefore, it becomes very difficult to align the light spot and the pinhole in both the optical axis direction and a planar direction perpendicular thereto. In the optical axis direction, the depth of the focus of the light spot 10 is about 140 µm. When the light spot 9 on the disc suffers a defocus of 1 µm, the focal point position of the light spot 10 is shifted by 200 µm corresponding to a value twice the longitudinal magnification, and falls outside the depth of the focus at the pinhole position. As a result, the side lobe cannot be effectively masked. In the planar direction perpendicular to the optical axis as well, when a light beam incident on the condenser lens is tilted by only 1 due to a change in temperature or aging, a half of the light spot 10 is undesirably masked by the pinhole 11, and as a result, signal reproduction is disabled. US-A-5206852 reflecting the preamble of claims 1,2 and 3 discloses an optical recording/reproducing apparatus in which a mask is inserted in the light path between the recording medium and the return light path from the optical recording medium. JP-A-02208835 and the corresponding Patent Abstracts of Japan, vol. 14, No. 504 (P-1127) and JP-A-61220146 and the corresponding Patent Abstracts of Japan, vol. 11, No. 056 (P-549) both disclose optical recording/reproduction apparatus in which a mask is arranged to intercept the peripheral edge of the reflected light from the surface of the recording medium. JP-A-63298831 and the corresponding Patent Abstracts of Japan, vol. 13, No. 129 (P-849) disclose a mask having a rhombic aperture which will be effective to limit the light beam in the direction of the track. The present invention has been made in consideration of the above situation, and has as its object to provide an optical recording/reproduction apparatus capable of high-density recording, which can effectively remove crosstalk components from an adjacent track and intersymbol interaction components from an adjacent mark in the track direction, which are included in a side lobe portion, can simplify the arrangement and adjustment, and may be made stable against a change in temperature and aging. In order to achieve the above object, according to the present invention, an optical recording/reproduction apparatus is arranged as follows. According to a first aspect of the present invention, there is provided an optical recording/reproduction apparatus, comprising: an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium having a plurality of neighboring tracks as a fine light spot so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system for detecting a returned light beam from the optical recording medium; and mask means, arranged in a far field region sufficiently separated from a focal plane of said detection optical system in an optical path of said detection optical system, for masking marginal rays, in a track direction, of the returned light beam, the apparatus being characterised in that said mask means satisfies the following relation: 0.77 - 0.1•(d2/m) < B2/A2 < 1.07 - 0.1·•(d2/m) for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot on the optical recording medium in the track direction, and m is the shortest mark length on the optical recording medium, whereby information reproduced by the light spot from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot is reduced. According to a second aspect of the present invention, there is provided an optical recording/reproduction apparatus comprising: an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium having a plurality of neighboring tracks as a fine light spot so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system for detecting a returned light beam from the optical recording medium; and mask means, arranged in a far field region sufficiently separated from a focal plane of said detection optical system in an optical path of said detection optical system, for masking marginal rays, in a track direction, of the returned light beam, wherein a dominant optical aberration of the light spot is a coma, the apparatus being characterised in that said mask means satisfies the following relation: 0.77 - 0.1•(d2/m) - 0.12•W31 < B2/A2 < 1.07 - 0.1• (d2/m) - 0.12•W31 for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot on the optical recording medium in the track direction, m is the shortest mark length on the optical recording medium, and W31 is the wave aberration coefficient of the coma whereby information reproduced by a side lobe, generated by the coma, of the light spot from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot is reduced. According to a third aspect of the present invention, there is provided an optical recording/reproduction apparatus comprising: an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium having a plurality of neighboring tracks as a fine light spot so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system for detecting a returned light beam from the optical recording medium; and mask means, arranged in a far field region sufficiently separated from a focal plane of said detection optical system in an optical path of said detection optical system, for masking marginal rays, in the track direction, of the returned light beam, wherein a dominant optical aberration of the light spot is a spherical aberration; the apparatus being characterised in that said mask means satisfies the following relation: 0.77 - 0.1•(d2/m) - 0.12•W40 2 < B2/A2 < 1.07 - 0.1• (d2/m) - 0.12•W40 2 for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot on the optical recording medium in the track direction, m is the shortest mark length on the optical recording medium, and W40 is the wave aberration coefficient of the spherical aberration, whereby information reproduced by a side lobe, generated by the spherical aberration, of the light spot from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot is reduced. A number of embodiments of the invention will now be described, by way of example only with reference to the accompanying drawings in which: Fig. 1 is a view for explaining a conventional optical system; Fig. 2 is a graph for explaining a light spot on an optical disc in the prior art shown in Fig. 1; Fig. 3 is a view for explaining another conventional optical system; Fig. 4 is a view for explaining a light spot on a photodetector in the prior art shown in Fig. 3; Fig. 5 is a view for explaining the principle of the invention according to the first comparative example; Fig. 6 is a view for explaining the principle of the invention according to the first comparative example; Fig. 7 is a front view for explaining an aperture according to the first comparative example; Fig. 8 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 9 is a graph for explaining the experimental results according to the first comparative example; Fig. 10 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 11 is a view for explaining the principle of the invention according to the first comparative example; Fig. 12 is a view for explaining the principle of the invention according to the first comparative example; Fig. 13 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 14 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 15 is a view for explaining an optical system according to the.first comparative example; Fig. 16 is a circuit diagram for explaining signal detection according to the first comparative example; Fig. 17 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 18 is a graph for explaining the simulation results of the effect of the invention according to the first comparative example; Fig. 19 to Fig. 33 are deleted Fig. 34 is a view for explaining the principle of the invention according to the first embodiment of the present invention; Fig. 35 is a view for explaining the principle of the invention according to the first embodiment of the present invention; Fig. 36 is a front view for explaining an aperture according to the first embodiment of the present invention; Fig. 37 is a graph for explaining the simulation results of the effect of the invention according to the first embodiment of the present invention; Fig. 38 is a graph for explaining the experimental results according to the first embodiment of the present invention; Fig. 39 is a view for explaining an optical system according to the first embodiment of the present invention; Fig. 40 is a circuit diagram for explaining signal detection according to the first embodiment of the present invention; Fig. 41 is a view for explaining the principle of the invention according to the second embodiment of the present invention; Fig. 42 is a view for explaining the principle of the invention according to the second embodiment of the present invention; Fig. 43 is a front view for explaining an aperture according to the second embodiment of the present invention; Fig. 44 is a view for explaining an optical system according to the second embodiment of the present invention; Fig. 45 is a circuit diagram for explaining signal detection according to the second embodiment of the present invention; Fig. 46 and Fig. 47 are deleted; Fig. 48 is a graph for explaining the intensity distribution of a light spot; Fig. 49 to Fig. 61 are deleted; Fig. 62 is a view for explaining the principle of the invention according to the third embodiment of the present invention; Fig. 63 is a view for explaining the principle of the invention according to the third embodiment of the present invention; Fig. 64 is a front view for explaining an aperture according to the third embodiment of the present invention; Fig. 65 is a graph for explaining the experimental results of the effect of the invention according to the third embodiment of the present invention; Fig. 66 is a graph for explaining the experimental results of the effect of the invention according to the third embodiment of the present invention; Fig. 67 is a graph for explaining the experimental results of the effect of the invention according to an embodiment of the present invention; Fig. 68 is a graph for explaining the experimental results of the effect of the invention according to an embodiment of the present invention; Fig. 69 is a front view for explaining the principle of the invention according to the fourth embodiment of the present invention; Fig. 70 is a front view for explaining the principle of the invention according to the fourth embodiment of the present invention; Fig. 71 is a graph for explaining the experimental results of the effect of the invention according to the fourth embodiment of the present invention; and Fig. 72 is a graph for explaining the experimental results of the effect of the invention according to the fourth embodiment of the present invention. The arrangement of a comparative example of an optical recording/reproduction apparatus according to the present invention will be described below with reference to Figs. 5 to 7. Fig. 5 is a sectional view, in the radial direction, of a magneto-optical disc 5 in an optical system of the present invention, Fig. 6 is a sectional view, in the track direction, of the disc 5, and Fig. 7 is a view showing the shape of an aperture 12. Figs. 5 and 6 show only a detection optical system (to be referred to as a light-receiving system hereinafter) for detecting returned light from the magneto-optical disc 5 to explain the principle of the present invention. Referring to Fig. 5, the magneto-optical disc 5 is tilted in the radial direction of the magneto-optical disc 5 indicated by an arrow 13, and a state wherein a side lobe due to a coma is generated in a light spot 9 by the tilt of the disc 5 is illustrated beside the light-receiving system. A light spot is illustrated in a state observed in the direction of rays, and its a-a' section corresponds to the radial direction. When information is reproduced from a given track in this state, a side lobe due to a coma undesirably reproduces information of an adjacent track, and the reproduced information is found in a reproduction signal as crosstalk components. In particular, in a disc consisting of a plastic material such as polycarbonate, a crosstalk problem from an adjacent track due to a tilt of the disc cannot be avoided, and seriously disturbs an increase in density. Returned light from the magneto-optical disc 5 is collimated via an objective lens 4, and is guided toward a photodetector 7 by a condenser lens 6. An aperture 12 is arranged in the optical path between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system so as to mask marginal rays in the radial direction, so that these rays do not reach the photodetector 7. Fig. 5 illustrates rays which pass various height positions of the pupil diameter. As can be seen from Fig. 5, of these rays, hatched rays masked by the aperture 12 mainly form a side lobe of a light spot on the disc. The aperture 12 is arranged in the far field region sufficiently separated from the focal plane of the light-receiving system and masks marginal rays, in the radial direction perpendicular to the track, of a returned light beam. Therefore, most of crosstalk components from an adjacent track are included in the marginal rays, and if these rays are masked, the crosstalk components can be reduced. The side lobe due to the coma and marginal rays in the vicinity of the pupil of the light-receiving system normally have a good correspondence therebetween although they do not have a strict one-to-one correspondence therebetween unlike that between the light spot on the disc and the light spot of the light-receiving system shown in Fig. 4. Referring to Fig. 6, the magneto-optical disc 5 is not tilted in the track direction indicated by an arrow 14. Similarly, Fig. 6 illustrates, beside the light-receiving system, a state wherein a side lobe due to a coma is generated in the light spot 9. The b-b' section of the light spot corresponds to the track direction. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 via the condenser lens 6. The aperture 12 has a dimension in the track direction larger than a beam diameter A so as not to mask marginal rays in the track direction. This is to guide modulated components of a reproduction signal from the disc to the photodetector as much as possible since these components are spatially distributed in the track direction in the pupil of the light-receiving system. Fig. 7 is a front view of the aperture 12. The dimension, in the radial direction, of the aperture is B1 (B1 < A1), and the dimension, in the track direction perpendicular to the radial direction, of the aperture is C1 (C1 > A1). If the objective lens 4 has a focal length fo = 3 mm and NA = 0.55, the beam diameter A1 = 3.3 mm. As will be described later, since the dimension B1 is selected to fall within a range of B1 ≃ 1.3 to 2.8 mm, easy adjustment of the aperture is greatly improved as compared to the prior art in which a pinhole having a diameter of 15 pm is inserted in the focal plane of the light-receiving system, and the aperture is not easily influenced by a change in temperature or aging. Since the aperture 12 is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. Figs. 8 to 10 show the results of computer simulations and experiments associated with a crosstalk reduction effect from an adjacent track in the optical system according to the first comparative example. Fig. 8 shows the calculation results of variations in crosstalk amount from an adjacent track and reproduction signal (carrier), which are obtained by varying a width B1, in the radial direction, of the aperture 12. The wavelength of the semiconductor laser is λ = 780 nm, the tilt of the disc is 5 mrad. in the radial direction, the NA of the objective lens is NA = 0.55, the track pitch is 1.4 µm, and the track width of the recorded portion is 0.9 µm. The mark length of the carrier is 0.75 µm, and the mark length of the crosstalk component recorded in the adjacent track is 3.0 µm. The abscissa represents the ratio of the width B1, in the radial direction, of the aperture 12 with respect to the beam diameter A1. The ordinate represents an amount normalized with the carrier or the crosstalk amount from the adjacent track when no aperture is arranged (B1/A1 = 1). When B1/A1 is varied, the reduction effect appears when B1/A1 = 0.85 or less. For example, when B1/A1 = 0.7, the decrease in carrier is about 1 dB, while the decrease in crosstalk amount from the adjacent track is about 5 dB. Thus, a remarkable effect is confirmed. Fig. 8 also shows the crosstalk amount to the carrier, which is normalized with a value obtained without an aperture, and a reduction effect of about 4 dB is confirmed. Fig. 9 shows the experimental results of a variation in C/N (carrier to noise) ratio of a reproduction signal (carrier), which are obtained by varying the width B1, in the radial direction of the disc, of the aperture 12. The wavelength of the semiconductor laser is λ = 780 nm, the tilt of the disc is 5 mrad. in the radial direction, the NA of the objective lens is NA = 0.55, the track pitch is 1.4 µm, and the track width of a recorded portion is 0.9 µm. The mark length of the carrier is 0.75 µm, and the mark length of the crosstalk component recorded in the adjacent track is 3.0 µm. The abscissa represents the ratio of the width B1, in the radial direction, of the aperture 12 with respect to the beam diameter A1. The ordinate represents the C/N ratio of the reproduction signal. As B1/A1 is decreased, the carrier is lowered, and the C/N ratio is lowered accordingly. In this case, the decrease in C/N ratio is smaller than that of the carrier (see Fig. 8). For example, when B1/A1 = 0.7, the decrease in carrier is about 1 dB, while the decrease in C/N ratio is about 0.5 dB. However, since the crosstalk amount to the carrier is largely reduced as compared to a case wherein no aperture is arranged, stable signal reproduction is assured as a whole. Fig. 10 shows the calculation results of the crosstalk amount from an adjacent track to the carrier, which are obtained by varying the tilt of the disc. The parameter used is the ratio of the width B1, in the radial direction, of the aperture 12 to the beam diameter A1. The abscissa represents the tilt of the disc, and the ordinate represents the crosstalk amount from an adjacent track to the carrier. As can be seen from Fig. 10, at any disc tilt, the reduction effect appears when B1/A1 ≃ 0.85, the reduction effect is enhanced as the disc tilt becomes small, and the reduction effect is gradually reduced as the disc tilt becomes large. For example, when B1/A1 = 0.73 and the disc is not tilted, a reduction effect of about 5 dB is expected. Another effect of the comparative example will be explained below with reference to Figs. 11 and 12. Fig. 11 is a sectional view, in the radial direction, of the magneto-optical disc 5 in the optical system of the present invention, and Fig. 12 is a sectional view, in the track direction, of the disc 5. Figs. 11 and 12 particularly illustrate only the light-receiving system for explaining the principle of the example. Fig. 11 also illustrates, beside the light-receiving system, a state wherein a side lobe due to a spherical aberration is generated in the light spot 9. The spherical aberration is generated due to a manufacturing error of the objective lens and a substrate thickness error of the disc, and has a side lobe which is symmetrical about the center of rotation. A light spot is illustrated in a state observed in the direction of rays, and its a-a' section corresponds to the radial direction. When information on a given track is reproduced in this state, the side lobe caused by the spherical aberration undesirably reproduces information on an adjacent track, and the reproduced information is found in a reproduction signal as crosstalk components. When the NA of the objective lens is to be increased, the allowable manufacturing error must be reduced, and the problem of crosstalk from an adjacent track due to a spherical aberration seriously disturbs an increase in density. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 by the condenser lens 6. The aperture 12 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, and masks marginal rays in the radial direction, so that they do not reach the photodetector 7. Fig. 11 illustrates rays which pass various height positions of the pupil diameter. As can be seen from Fig. 11, of these rays, hatched rays masked by the aperture 12 mainly form a side lobe of a light spot on the disc. Therefore, most of crosstalk components from the adjacent track are included in the marginal rays, and the crosstalk can be reduced by masking these rays. The side lobe caused by the spherical aberration and the marginal rays in the vicinity of the pupil of the light-receiving system normally have a good correspondence therebetween as in the case of a coma. Referring to Fig. 12, an arrow 14 and the b-b' section of a light spot correspond to the track direction perpendicular to the radial direction. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 by the condenser lens 6. The aperture 12 has a dimension, in the track direction, larger than the beam diameter A1 so as not to mask marginal rays in the track direction for the same reason as in Fig. 5. Figs. 13 and 14 show the computer simulation results of the reduction effect of crosstalk components from an adjacent track in the optical system according to the first comparative example. Fig. 13 shows the calculation results of the crosstalk from an adjacent track to the carrier, which are obtained when the width B1, in the radial direction, of the apertures 12 is set to be B1/A1 = 0.73 and various spherical aberrations are generated. The spherical aberration (W40) was varied to ±0.53λ, ±0.40λ, ±0.27λ, and ±0.0λ. Other calculation conditions are the same as those in Figs. 8 and 10. The abscissa represents the defocus, and the ordinate represents the crosstalk amount from an adjacent track to the carrier. Fig. 14 shows similar calculation results obtained when no aperture is arranged (B1/A1 = 1) and various spherical aberrations are generated, for the sake of comparison. As can be seen from a comparison between Figs. 13 and 14, the crosstalk reduction effect can be obtained for every spherical aberrations and defocus amounts. As can also be seen from Figs. 13 and 14, as the spherical aberration becomes small, the reduction effect is enhanced; as the spherical aberration becomes large, the effect is reduced. For example, when the spherical aberration = ±0.27λ, a reduction effect of about 4 dB is obtained; when a spherical aberration = ±0.53λ, a reduction effect of about 2 dB is obtained. Figs. 15 and 16 show the entire magneto-optical head optical system according to the first comparative example. Fig. 15 is a front view of a magneto-optical disc recording/reproduction apparatus of the present invention, and Fig. 16 is a circuit diagram for explaining a method of detecting a magneto-optical signal and servo signals. The same reference numerals in Figs. 15 and 16 denote the same parts as in Fig. 5, and a detailed description thereof will be omitted. The magneto-optical head optical system shown in Fig. 15 is a so-called separated optical system. A light beam emitted from the semiconductor laser 1 is collimated by the collimator lens 2. The light beam incident on the polarization beam splitter 3 emerges from a stationary portion optical system 29 toward an optical head movable portion 16, and forms the fine light spot 9 on the magneto-optical disc 5 by the objective lens 4. The optical head movable portion 16 carries the objective lens 4, an actuator for driving the lens 4, a mirror 15, and the like. In order to shorten the access time to a predetermined track, the movable portion is constituted by minimum required parts, and a semiconductor laser light source portion, a signal detection system, and the like, which have a large weight, are arranged on the stationary portion optical system 29. The light beam, which is reflected by the recording medium surface and is incident again on the objective lens 4, is reflected by the polarization beam splitter 3 via the mirror 15, and is guided toward a signal detection system. The light beam transmitted through a halfwave plate 17 passes through the aperture 12 of the comparative example, and some marginal rays in the radial direction are masked. The aperture 12 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, as described above. The light beam transmitted through the condenser lens 6 and a cylindrical lens 19 is transmitted through or reflected by a polarization beam splitter 18, and is guided onto photodetectors 7-1 and 7-2. In an information reproduction mode, the semiconductor laser 1 emits low-power light, and reproduction of a magneto-optical signal and detection of servo signals are performed. In a recording mode, the semiconductor laser 1 emits high-power light. Upon irradiation of a high-power laser beam, the temperature of the recording medium surface of the magneto-optical disc 5 rises, and the magnetization and the coercive force are lowered. Therefore, when a magnetic head 50 applies a magnetic field whose polarity is inverted in correspondence with recording information to the disc, a magneto-optical pit is recorded. The detection system of a magneto-optical signal and servo signals will be described below with reference to Fig. 16. Fig. 16 illustrates a state wherein the light beam reflected by the polarization beam splitter 3 forms light spots 20-1 and 20-2 on the photodetectors 7-1 and 7-2 via the condenser lens 6 and the cylindrical lens 19. In order to detect a focusing error signal by an astigmatism method, the photodetector 7-1 is arranged in the vicinity of the circle of least confusion, and a light spot on the photodetector normally has a circular shape. In this case, however, since marginal rays in the radial direction are masked by the aperture 12, the light spot has a shape as shown in Fig. 16. Sums of diagonal photoelectric conversion outputs of the light spot 20-1 on the photodetector 7-1 are calculated, and a difference between the sums is differentially amplified by a differential amplifier 21 to generate a focusing error signal 26. A difference, in the radial direction, of the outputs of the light spot 20-2 on the photodetector 7-2 is differentially amplified by a differential amplifier 22 to generate a tracking error signal (push-pull signal) 27. A magneto-optical signal 28 is detected in such a manner that the sum outputs of the photodetectors 7-1 and 7-2 are generated by sum amplifiers 23 and 24, and thereafter, a difference therebetween is differentially amplified by a differential amplifier 25. Note that the photodetector 7-1 can simultaneously obtain a tracking error signal from the differential output in the radial direction since it adopts a quadrant sensor. Since the optical system shown in Fig. 15 adopts a single detection system to detect the magneto-optical signal and the servo signals, some light components, in the track direction, of a light beam are masked by the aperture 12, and the amplitude of a tracking signal is undesirably lowered. Figs. 17 and 18 show the computer simulation results of the influence of the aperture 12 on the tracking signal. Fig. 17 shows the calculation results of a variation in amplitude of the tracking signal, which are obtained by varying the width B1, in the radial direction, of the aperture 12. The calculation conditions are the same as those in Figs. 8 and 10. The abscissa represents the radio of the width B1, in the radial direction, of the aperture 12 to the beam diameter A1. The ordinate represents the amount normalized with the amplitude of the tracking signal obtained when no aperture is arranged (B1/A1 = 1). As B1/A1 becomes smaller, the amplitude of the tracking signal is lowered quadratically. For example, when B1/A1 = 0.7, the amplitude of the tracking signal is about 70% of that obtained when no aperture is arranged. This reveals that rays masked to reduce crosstalk components from an adjacent track include components modulated by a groove crossing signal. Fig. 18 shows the calculation results of a variation in offset of the tracking signal, which are obtained when the width B1, in the radial direction, of the aperture 12 is varied and the objective lens is shifted in the radial direction. When a relatively nearby track is to be accessed, the objective lens is often shifted in the radial direction. The abscissa represents the objective lens shift amount normalized with the effective diameter (beam diameter A1) of the objective lens, and the ordinate represents the offset of the tracking signal. As B1/A1 becomes smaller, the offset of the tracking signal is reduced. For example, when B1/A1 is set to be about 0.7, almost no offset of the tracking signal is generated even when the objective lens is shifted in the radial direction. Although this value slightly varies depending on the groove shapes of discs, the same tendencies are observed. This is because even when the objective lens is shifted in the radial direction, the movements of the light spots 20-1 and 20-2 on the photodetectors 7-1 and 7-2 are limited by the aperture 12. As described above, when some rays in the radial direction are masked by the aperture 12 and it influences the servo signals since a single detection system is used to detect the magneto-optical signal and the servo signals like in the optical system shown in Fig. 14, the aperture 12 preferably has B1/A1 = 0.50 to 0.85. More preferably, the aperture 12 has B1/A1 = 0.55 to 0.75. When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, crosstalk components from an adjacent track can be effectively reduced, and a decrease in amplitude of the tracking signal can be suppressed within an allowable range. In addition, even when the objective lens is shifted in the radial direction, the offset of the tracking signal can be decreased. The arrangement of a first embodiment according to the present invention will be described below with reference to Figs. 34 to 36. Fig. 34 is a sectional view, in the track direction, of a magneto-optical disc 5 in an optical system of the present invention, Fig. 35 is a sectional view, in the radial direction, of the disc 5, and Fig. 36 is a view showing the shape of an aperture 38. Figs. 34 and 35 particularly illustrate only the light-receiving system for explaining the principle of the present invention. Referring to Fig. 34, the magneto-optical disc 5 is tilted in the track direction indicated by an arrow 14, and a state wherein a side lobe due to a coma is generated in a light spot 9 by the tilt of the disc 5 is illustrated beside the light-receiving system. A light spot is illustrated in a state observed in the direction of rays, and its b-b' section corresponds to the track direction. When information on a given track is reproduced in this state, the side lobe caused by the coma undesirably reproduces information of an adjacent mark, and the reproduced information generates a distortion or shift in a reproduction signal to be originally reproduced. This is called an intersymbol interaction, and causes an increase in jitter. In particular, in a disc consisting of a plastic material such as polycarbonate, the problem of an increase in jitter caused by a tilt of the disc is unavoidable, and seriously disturbs an increase in density. Returned light from the magneto-optical disc 5 is collimated via an objective lens 4, and is guided to a photodetector 7 by a condenser lens 6. An aperture 38 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, and masks marginal rays in the track direction, so that these rays do not reach the photodetector. Fig. 34 illustrates rays which pass various height positions of the pupil diameter. As can be seen from Fig. 34, of these rays, hatched rays masked by the aperture 38 mainly form a side lobe of a light spot on the disc. Therefore, most of information components of an adjacent mark are included in these marginal rays, and the intersymbol interaction can be reduced by masking these rays. The side lobe due to the coma and marginal rays in the vicinity of the pupil of the light-receiving system normally have a good correspondence therebetween although they do not have a strict one-to-one correspondence therebetween unlike that between the light spot on the disc and the light spot of the light-receiving system shown in Fig. 4. The aperture 38 is arranged in the far field region sufficiently separated from the focal plane of the light-receiving system, and masks marginal rays, in the track direction, of the returned light beam. Referring to Fig. 35, the magneto-optical disc 5 is not tilted in the track direction indicated by the arrow 14. Similarly, Fig. 35 illustrates, beside the light-receiving system, a state wherein a side lobe due to a coma is generated in the light spot 9. The a-a' section of the light spot corresponds to the radial direction. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 via the condenser lens 6. The aperture 38 has a dimension in the radial direction larger than the beam diameter A2 so as not to mask marginal rays in the radial direction. This is to guide modulated components of a tracking signal from the disc to the photodetector as much as possible since these components are spatially distributed in the radial direction in the pupil of the light-receiving system. Fig. 36 is a front view of the aperture 38. The dimension, in the track direction, of the aperture is B2 (B2 < A2), and the dimension, in the radial direction, of the aperture is C2 (C2 > A2). If the objective lens 4 has a focal length fo = 3 mm and NA = 0.55, the beam diameter A2 = 3.3 mm. As will be described later, since the dimension B2 is selected to fall within a range of B2 = 1.8 to 3.0 mm, easy adjustment of the aperture is greatly improved as compared to the prior art in which a pinhole having a diameter of 15 µm is inserted in the focal plane of the light-receiving system, and the aperture is not easily influenced by a change in temperature or aging. Since the aperture 38 is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. From equation (1) described above, the aperture 38 can be arranged at a position separated from the focal plane of the light-receiving system by a distance d given by the following equation: d = 8•(FNo) 2 •λ where FNo is the effective f-number of the light-receiving system, and λ is the wavelength of a light beam from a light source. Fig. 37 shows the computer simulation results of a decrease in carrier in the optical system according to the seventh embodiment of the present invention. Fig. 37 shows the calculation results of a change in reproduction signal (carrier) by changing the width B2, in the track direction, of the aperture 38. The wavelength λ of the semiconductor laser is set to be λ = 780 nm, the tilt of the disc is 5 mrad. in the track direction, the objective lens has NA = 0.55, the track pitch is 1.4 µm, the track width of the recorded portion is 0.9 µm, and the carrier has a mark length = 0.75 µm. The abscissa represents the ratio of the width 82, in the track direction, of the aperture 38 to the beam diameter A2. The ordinate represents the amount normalized with a carrier obtained when no aperture is arranged (B2/A2 = 1). When B2/A2 is decreased, the carrier level is decreased gradually. This decrease in carrier is larger than that in a case wherein the width, in the radial direction, of the aperture is changed, as shown in Fig. 8. This is because modulated components of a reproduction signal from a disc are spatially distributed in the track direction in the pupil of the light-receiving system. Fig. 38 shows the experimental results of the jitter reduction effect for information (intersymbol interaction) reproduced from an adjacent mark on a single track in the optical system according to the first embodiment of the present invention. The experimental conditions are: a mark edge recording method, a 1-7 modulation method for symbols, a minimum mark length = 0.75 µm, and a linear velocity = 15 m/s. Fig. 38 shows cases obtained when the disc is not tilted and when the disc is tilted at 5 mrad. in the track direction. The abscissa represents the ratio of the width B2, in the track direction, of the aperture 38 to the beam diameter A2, and the ordinate represents the jitter amount. At no disc tilt, the reduction effect is maximized when B2/A2 is set to be about 0.75, and the effect is gradually reduced when B2/A2 is set to be equal to or lower than 0.75. This is for the following reason. That is, although the aperture 38 masks information from an adjacent mark, since the carrier is lowered as B2/A2 becomes smaller, the C/N ratio deteriorates, and the level of jitter components due to noise increases. At 5 mrad. disc tilt, the reduction effect is maximized when B/A is set to be equal 0.72, which is rather lower than at no disc tilt, and the effect is gradually reduced in the same manner. Figs. 39 and 40 show the entire magneto-optical head optical system according to the first embodiment of the present invention. Fig. 39 is a front view of a magneto-optical disc recording/reproduction apparatus of the present invention, and Fig. 40 is a circuit diagram for explaining a method of detecting a magneto-optical signal and servo signals. The same reference numeral in Fig. 39 denote parts having the same functions in Fig. 15, and a detailed description thereof will be omitted. The magneto-optical head optical system shown in Fig. 39 is a so-called separated optical system. A light beam emitted from the semiconductor laser 1 is collimated by the collimator lens 2. The light beam incident on the polarization beam splitter 3 emerges from a stationary portion optical system 29 toward an optical head movable portion 16, and forms the fine light spot 9 on the magneto-optical disc 5 by the objective lens 4. The light beam, which is reflected by the recording medium surface and is incident again on the objective lens 4, is reflected by the polarization beam splitter 3 via a mirror 15, and is guided toward a signal detection system. The light beam transmitted through a halfwave plate 17 passes through the aperture 38 of the present invention, and some marginal rays in the track direction are masked. The aperture 38 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, as described above. The light beam transmitted through the condenser lens 6 and a cylindrical lens 19 is transmitted through or reflected by a polarization beam splitter 18, and is guided onto photodetectors 7-1 and 7-2. A system for reproducing a magneto-optical signal and detecting servo signals will be explained below with reference to Fig. 40. Fig. 40 illustrates a state wherein the light beam reflected by the polarization beam splitter 3 forms light spots 20-1 and 20-2 on the photodetectors 7-1 and 7-2 via the condenser lens 6 and the cylindrical lens 19. In order to detect a focusing error signal by an astigmatism method, the photodetector 7-1 is arranged in the vicinity of the circle of least confusion, and a light spot on the photodetector normally has a circular shape. In this case, however, since marginal rays in the track direction are masked by the aperture 38, the light spot has a shape as shown in Fig. 40. Sums of diagonal photoelectric conversion outputs of the light spot 20-1 on the photodetector 7-1 are calculated, and a difference between the sums is differentially amplified by a differential amplifier 21 to generate a focusing error signal 26. A difference, in the radial direction, of the outputs of the light spot 20-2 on the photodetector 7-2 is differentially amplified by a differential amplifier 22 to generate a tracking error signal (push-pull signal) 27. A magneto-optical signal 28 is detected in such a manner that the sum outputs of the photodetectors 7-1 and 7-2 are generated by sum amplifiers 23 and 24, and thereafter, a difference therebetween is differentially amplified by a differential amplifier 25. Note that the photodetector 7-1 can simultaneously obtain a tracking error signal from the differential output in the radial direction since it adopts a quadrant sensor. Note that the optical system shown in Fig. 39 does not suffer a decrease in amplitude of a tracking signal which poses a problem in the optical system shown in Fig. 15 since rays in the radial direction are not masked although a single detection system is used for detecting a magneto-optical signal and servo signals. As described above, in this embodiment, although some rays in the track direction are masked by the aperture 38, they do not influence servo signals. Therefore, regardless of whether a single detection signal is used or independent detection systems are used for detecting a magneto-optical signal and servo signals, the aperture 38 preferably has B2/A2 = 0.55 to 0.90. More preferably, the aperture 38 has B2/A2 = 0.60 to 0.85. When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, information from an adjacent mark is masked, and jitter components due to an intersymbol interaction can be reduced. As described above, when marginal rays in the track direction of the light-receiving system are masked, information from an adjacent mark can be masked, and jitter components due to an intersymbol interaction can be reduced. As in the first comparative example in which marginal rays in the radial direction are masked to reduce crosstalk components from the adjacent track, means for masking marginal rays in the track direction may be arranged in the vicinity of the pupil of the light-receiving system or in the far field region sufficiently separated from the focal point of the light-receiving system. The shape of the aperture 38 may be a rectangle or an ellipse. Also, marginal rays in the track direction may be prevented from being received by modifying the shape of the photodetectors. Furthermore, they can be applied to an optical system which includes a single detection, system for a magneto-optical signal and servo signals, and an optical system which includes independent detection systems therefor. Note that reduction of jitter components caused by a coma due to a disc tilt has been particularly exemplified. However, this embodiment is effective for a case wherein a spherical aberration or a defocus is generated. When the present invention is adopted, easy adjustment of the aperture is greatly improved as, compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and this arrangement is not easily influenced by a change in temperature or aging. Since the aperture is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. The arrangement according to the second embodiment of the present invention will be described below with reference to Figs. 41 to 43. Fig. 41 is a sectional view, in the track direction, of a magneto-optical disc 5 in an optical system of the present invention, Fig. 42 is a sectional view, in the radial direction, of the disc 5, and Fig. 43 is a view showing the shape of an aperture 39. Figs. 41 and 42 particularly illustrate only the light-receiving system for explaining the principle of the present invention. Fig. 41 illustrates, beside the light-receiving system, a state wherein a side lobe due to a spherical aberration is generated in a light spot 9. The spherical aberration is generated due to a manufacturing error of the objective lens and a substrate thickness error of the disc, and has a side lobe which is symmetrical about the center of rotation. A light spot is illustrated in a state observed in the direction of rays, and its a-a' section corresponds to the radial direction. When information on a given track is reproduced in this state, the side lobe caused by the spherical aberration undesirably reproduces information on an adjacent track, and the reproduced information is found in a reproduction signal as crosstalk components. When the NA of the objective lens is to be increased, the manufacturing allowable error must be reduced, and the problem of the crosstalk from an adjacent track due to a spherical aberration seriously disturbs an increase in density. Fig. 42 similarly illustrates, beside the light-receiving system, a state wherein a side lobe due to a spherical aberration is generated in the light spot 9. The light spot is illustrated in a state observed in the direction of rays, and its b-b' section corresponds to the track direction. When information on a given track is reproduced in this state, the side lobe caused by the spherical aberration undesirably reproduces information on an adjacent mark, and the reproduced information generates a distortion or shift in a reproduction signal to be originally reproduced. Thus, the intersymbol interaction undesirably increases, which, in turn, increases jitter. When the NA of the objective lens is to be increased, the manufacturing allowable error must be reduced, and the problem of the increase in jitter due to a spherical aberration is unavoidable, thus seriously disturbing an increase in density. Returned light from the magneto-optical disc 5 is collimated via an objective lens 4, and is guided to a photodetector 7 by a condenser lens 6. The aperture 39 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, and masks marginal rays in the radial and track directions, so that these rays do not reach the photodetector 7. Figs. 41 and 42 illustrate rays which pass various height positions of the pupil diameter. As can be seen from Figs. 41 and 42, of these rays, hatched rays masked by the aperture 39 mainly form a side lobe of a light spot on the disc. Therefore, most of crosstalk components from an adjacent track are included in marginal rays in the radial direction, and can be reduced by masking these rays. On the other hand, most of information components from an adjacent mark are included in the marginal rays in the track direction, and the intersymbol interaction can be reduced by masking these rays. Fig. 43 is a front view of the aperture 39. The dimension, in the radial direction, of the aperture is B1 (B1 < A1), and the dimension, in the track direction, of the aperture is C1 (C1 < A1). If the objective lens 4 has a focal length to = 3 mm and NA = 0.55, the beam diameter A1 = 3.3 mm. When B1 and C1 are selected to respectively fall within a range of B1 ≃ 1.3 to 2.8 mm and a range of C1 ≃ 1.8 to 3.0 mm, crosstalk components from an adjacent track and jitter components due to intersymbol interaction from an adjacent mark can be simultaneously reduced. In this embodiment, easy adjustment of the aperture is greatly improved as compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and the aperture is not easily influenced by a change in temperature or aging. Since the aperture 39 is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. Figs. 44 and 45 show the entire magneto-optical head optical system according to the eighth embodiment of the present invention. Fig. 44 is a front view of a magneto-optical disc recording/reproduction apparatus of the present invention, and Fig. 45 is a circuit diagram for explaining a method of detecting a magneto-optical signal. The same reference numeral in Fig. 44 denote parts having the same functions as in Fig. 15, and a detailed description thereof will be omitted. Referring to Fig. 44, a light beam emitted from the semiconductor laser 1 is collimated by a collimator lens 2. A light beam incident on a polarization beam splitter 3 emerges from a stationary portion optical system 29 toward an optical head movable portion 16, and forms a fine light spot 9 on the magneto-optical disc 5 via the objective lens 4. The light beam, which is reflected by the recording medium surface and is incident again on the objective lens 4, is reflected by the polarization beam splitter 3 via a mirror 15, and is guided toward a signal detection system. A polarization beam splitter 30 splits the optical path into two paths for a magneto-optical signal detection system and a servo signal detection system. The light beam reflected by the polarization beam splitter 30 is guided toward a photodetector 31 via the condenser lens 6 and a cylindrical lens 19. The photodetector 31 comprises a quadrant sensor, and obtains a focusing error signal and a tracking error signal by the arrangement shown in Fig. 16 (not shown). Since the aperture 39 is not arranged between the objective lens 4 and the condenser lens 6, a normal circular light spot is formed on the photodetector 31. The light beam transmitted through the polarization beam splitter 30 passes through the aperture 39 of the present invention via a halfwave plate 17, and some marginal rays in the radial and track directions are masked. The aperture 39 is arranged in the vicinity of the pupil of the light-receiving system, as described above. The light beam transmitted through or reflected by a polarization beam splitter 18 is guided onto photodetectors 7-1 and 7-2. Reproduction of a magneto-optical signal will be explained below with reference to Fig. 45. Fig. 45 illustrates a state wherein the light beam reflected by the polarization beam splitter 18 forms light spots 20-1 and 20-2 on the photodetectors 7-1 and 7-2. Since marginal rays in the radial and track directions are masked by the aperture 39, a light spot shape shown in Fig. 41 is obtained. A magneto-optical signal 28 is detected by differentially amplifying sum outputs from the photodetectors 7-1 and 7-2 by a differential amplifier 25. As in an optical system shown in Fig. 44, when the aperture 39 does not influence servo signals since independent detection systems are arranged to detect a magneto-optical signal and servo signals, the aperture 39 preferably has B1/A1 = 0.40 to 0.85. More preferably, the aperture 39 has B1/A1 = 0.45 to 0.75. Also, the aperture 39 preferably has C1/A1 = 0.55 to 0.90. More preferably, the aperture 39 has C1/A1 = 0.60 to 0.85. When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, crosstalk components from an adjacent track and information from an adjacent mark are masked, and crosstalk components and jitter components due to an intersymbol interaction can be reduced. As described above, when marginal rays in the radial and track directions of the light-receiving system are masked, crosstalk components from an adjacent track and information from an adjacent mark are masked, and crosstalk components and jitter components due to an intersymbol interaction can be reduced. Means for masking marginal rays in the track and radial directions may be arranged in the vicinity of the pupil of the light-receiving system or in the far field region sufficiently separated from the focal point of the light-receiving system. The shape of the aperture 39 may be a rectangle or an ellipse. Also, marginal rays in the track and radial directions may be prevented from being received by modifying the shape of the photodetectors. Furthermore, they can be applied to an optical system which includes a single detection system for a magneto-optical signal and servo signals, and an optical system which includes independent detection systems therefor. Note that reduction of jitter components caused by a spherical abberation has been particularly exemplified. However, this embodiment is effective for a case wherein a coma or a defocus is generated. When the present invention is adopted, easy adjustment of the aperture or photodetector is greatly improved as compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and this arrangement is not easily influenced by a change in temperature or aging. Since the aperture is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. As described above, when an aperture for masking marginal rays, in the radial direction, of returned light from a disc is arranged in a region sufficiently separated from the focal plane of an optical head light-receiving system, e.g., in the vicinity of the pupil of the light-receiving system, the influence of crosstalk from an adjacent track can be effectively reduced. In addition, the same effect can be expected when marginal rays, in the radial direction, of returned light are masked by modifying the shape of the photodetector. Also, when an aperture for masking marginal rays, in the track direction, of returned light from a disc is arranged in a region sufficiently separated from the focal plane of an optical head light-receiving system, e.g., in the vicinity of the pupil of the light-receiving system, information from an adjacent mark can be masked, and jitter components due to an intersymbol interaction can be reduced. In addition, the same effect can be expected when marginal rays, in the track direction, of returned light are masked by modifying the shape of the photodetector. The present invention can be widely applied to both an optical system which includes a single detection system for detecting a magneto-optical system and servo signals, and an optical system which includes independent detection systems therefor. When the present invention is adopted, easy adjustment of the aperture or photodetector is greatly improved as compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and this arrangement is not easily influenced by a change in temperature or aging. Since the aperture is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. Since crosstalk components from an adjacent track or intersymbol interaction factors from a mark adjacent in the track direction, which are included in a side lobe portion, can be effectively reduced by inserting a simple aperture or by modifying the shape of the photodetector, an optical head can have a compact structure. In addition, since easy adjustment of the aperture or photodetector is also improved, cost can be reduced. According to the present invention, when recording/reproduction of information or reproduction of information is performed by irradiating a light beam from a light source as a fine light spot onto a predetermined track of an optical recording medium having a plurality of neighboring tracks, mask means for masking marginal rays, in a direction perpendicular to the track, of a returned light beam from the optical recording medium is arranged in the far field region sufficiently separated from the focal plane of a detection optical, system for detecting the returned light beam from the optical recording medium in an optical path of the detection optical system, thereby reducing information (crosstalk components from an adjacent track) reproduced from the adjacent track upon reproduction of information from the predetermined track. The mask means for masking the light beam preferably has an opening having a ratio of the width of the opening to the beam diameter of the returned light beam in the direction perpendicular to the track, which ratio falls within a range from 0.4 to 0.9. According to the present invention, when recording/reproduction of information or reproduction of information is performed by irradiating a light beam from a light source as a fine light spot onto a predetermined track of an optical recording medium, mask means for masking marginal rays, in the track direction, of a returned light beam from the optical recording medium is arranged in the far field region sufficiently separated from the focal plane of a detection optical system for detecting the returned light beam from the optical recording medium in an optical path of the detection optical system, thereby reducing information (intersymbol interaction) reproduced from an adjacent mark located on a single track upon reproduction of a predetermined track located on the track. The mask means for masking the light beam preferably has an opening having a ratio of the width of the opening to the beam diameter of the returned light beam in the track direction, which ratio falls within a range from 0.55 to 0.9. The ratios of the aperture width to the beam diameter of the returned light were calculated under the following conditions. Upon evaluation of crosstalk components from an adjacent track, the wavelength of the semiconductor laser was set to be λ = 780 nm, the NA of the objective lens was set to be 0.55, the track pitch was set to be 1.4 µm, the track width of the recorded portion was set to be 0.9 µm, the mark length of the carrier was set to be 0.75 µm, and the mark length of crosstalk components recorded on the adjacent track was set to be 3.0 µm. Upon evaluation of information (intersymbol interaction) reproduced from an adjacent mark on a single track, a mark edge recording method with a minimum mark length = 0.75 µm and a 1-7 modulation method for symbols were used. However, it was found from more detailed simulations that the crosstalk amount from an adjacent track was an amount associated with the light spot diameter determined based on the NA of the objective lens and the wavelength of the semiconductor laser in the optical head, and the track pitch if neither a tilt of the disc substrate nor a substrate thickness error occurred (in other words, when the coma and the spherical aberration were satisfactorily corrected). Thus, the ratio of the opening width to the beam diameter of the returned light includes these parameters. It was also found that the crosstalk amount from an adjacent track was an amount associated with the wave aberration coefficient of a coma when, e.g., the tilt of the disc substrate was noticeable and a coma was generated. Similarly, it was found that the crosstalk amount from an adjacent track was an amount associated with the wave aberration coefficient of a spherical aberration when, e.g., the substrate thickness error was noticeable and a spherical aberration was generated. Furthermore, it was found that the intersymbol interaction amount was an amount associated with the light spot diameter and the minimum mark length, which were determined based on the NA of the objective lens and the wavelength of the semiconductor laser in the optical head, if neither a tilt of the disc substrate nor a substrate thickness error occurred. It was found that the intersymbol interaction amount was an amount associated with the wave aberration coefficient of a coma when, e.g., the tilt of the disc substrate was noticeable and a coma was generated. Similarly, it was found that the intersymbol interaction amount was an amount associated with the wave aberration coefficient of a spherical aberration when, e.g., the substrate thickness error was noticeable and a spherical aberration was generated. According to the present invention described in embodiments to be described below, when recording/reproduction of information or reproduction of information is performed by irradiating a light beam from a light source as a fine light spot onto a predetermined track of an optical recording medium having a plurality of neighboring tracks, mask means for masking marginal rays, in a direction perpendicular to the track, of a returned light beam from the optical recording medium is arranged in the far field region sufficiently separated from the focal plane of a detection optical system for detecting the returned light beam from the optical recording medium in an optical path of the detection optical system, thereby reducing information (crosstalk components from an adjacent track) reproduced from the adjacent track upon reproduction of information from the predetermined track. The mask means is set to satisfy the following relations. When a reproduction signal of information and a tracking servo signal are generated from a single photodetector in a state wherein the coma and the spherical aberration are satisfactorily corrected, the optimal ratio (B1/A1) of the aperture width B1 of the mask means to the beam diameter A1 of the returned light beam satisfies: 0.74 - 0.21•(d1/p) < B1/A1 < 1.09 - 0.21•(d1/p) where d1 is the 1/e 2 diameter (defined by 1/e 2 of the central intensity) of the light spot of the optical recording medium in the direction perpendicular to the track and p is the track pitch of the optical recording medium. When a reproduction signal of information and a tracking servo signal are generated from independent photodetectors in a state wherein the coma and the spherical aberration are satisfactorily corrected, the ratio (B1/A1) satisfies: 0.64 - 0.21•(d1/p) < B1/A1 < 1.09 - 0.21•(d1/p) When a reproduction signal of information and a tracking servo signal are generated from a single photodetector in a state wherein the coma is dominant, the ratio (B1/A1) satisfies: 0.74 - 0.21•(d1/p) - 0.25•W31 < B1/A1 < 1.09 - 0.21•(d1/p) - 0.25•W31 where W31 is the wave aberration coefficient of the coma. When a reproduction signal of information and a tracking servo signal are generated from independent photodetectors in a state wherein the coma is dominant, the ratio (B1/A1) satisfies: 0.64 - 0.21•(d1/p) - 0.25•W31 < B1/A1 < 1.09 - 0.21•(d1/p) - 0.25•W31 where W31 is the wave aberration coefficient of the coma. When a reproduction signal of information and a tracking servo signal are generated from a single photodetector in a state wherein the spherical aberration is dominant, the ratio (B1/A1) satisfies: 0.74 - 0.21•(d1/p) - 0.26•W40 2 < B1/A1 < 1.09 - 0.21•(d1/p) - 0.26•W40 2 where W40 is the wave aberration coefficient of the spherical aberration. When a reproduction signal of information and a tracking servo signal are generated from independent photodetectors in a state wherein the spherical aberration is dominant, the ratio (B1/A1) satisfies: 0.64 - 0.21•(d1/p) - 0.26•W40 2 < B1/A1 < 1.09 - 0.21•(d1/p) - 0.26•W40 2 where W40 is the wave aberration coefficient of the spherical aberration. According to the present invention described in embodiments to be described below, when recording/reproduction of information or reproduction of information is performed by irradiating a light beam from a light source as a fine light spot onto a predetermined track of an optical recording medium, mask means for masking marginal rays, in the track direction, of a returned light beam from the optical recording medium is arranged in the far field region sufficiently separated from the focal plane of a detection optical system for detecting the returned light beam from the optical recording medium in an optical path of the detection optical system, thereby reducing information (intersymbol interaction) reproduced from an adjacent mark on a single track upon reproduction of a predetermined mark located on the track. The mask means is set to satisfy the following relations. When the coma and the spherical aberration are satisfactorily corrected, the optimal ratio (B2/A2) of the aperture width B2 of the mask means to the beam diameter A2 of the returned light beam satisfies: 0.77 - 0.1•(d2/m) < B2/A2 < 1.07 - 0.1•(d2/m) where d2 is the 1/e 2 diameter (defined by 1/e 2 of the central intensity) of the light spot of the optical recording medium in the track direction and m is the minimum mark length on a disc. When the coma is dominant, the ratio (B2/A2) satisfies: 0.77 - 0.1•(d2/m) - 0.12•W31 < B2/A2 < 1.07 - 0.1•(d2/m) - 0.12•W31 where W31 is the wave aberration coefficient of the coma. When the spherical aberration is dominant, the ratio (B2/A2) satisfies: 0.77 - 0.1•(d2/m) - 0.12•W40 2 < B2/A2 < 1.07 - 0.1•(d2/m) - 0.12•W40 2 where W40 is the wave aberration coefficient of the spherical aberration. Still other embodiments of the present invention will be described hereinafter. The arrangement according to the third embodiment of the present invention will be described below with reference to Figs. 62 to 64. Fig. 62 is a sectional view, in the track direction, of a magneto-optical disc 5 in an optical system of the present invention, Fig. 63 is a sectional view, in the radial direction, of the disc 5, and Fig. 64 is a view showing the shape of an aperture 38. Figs. 62 and 63 particularly illustrate only an optical system for receiving returned light from a disc so as to explain the principle of the present invention. Referring to Figs. 62 and 63, an objective lens 4 forms a fine light spot 9 on the information recording surface of the magneto-optical disc 5. Assume that the light spot 9 is focused in an ideal state, and the coma and the spherical aberration are satisfactorily corrected. Referring to Fig. 62, an arrow 14 indicates the track direction, and a plurality of marks are aligned in the direction of the arrow 14. A in Fig. 62 represents the width of a light beam of the most marginal portion of an optical system for forming a light spot on a disc, and this width A is determined by an aperture 4' of an objective lens. The width A represents the NA of the light projection system of the objective lens. Fig. 62 also illustrates, beside the light-receiving system, a state wherein a side lobe due to the influence of the aperture 4' of the objective lens is generated in the light spot 9. The light spot is illustrated in a state observed in the direction of rays, and its b-b' section corresponds to the track direction. The light intensity distribution of the light spot 9 in the b-b' section is the same as that shown in Fig. 48, and a side lobe having an intensity as high as about 2% of the central intensity is generated. When information of a mark on a given track is reproduced in this state, the side lobe due to the influence of the aperture of the objective lens reproduces information of an adjacent mark on this track, and the reproduced information generates a distortion or shift in a reproduction signal to be originally reproduced. This is called an intersymbol interaction, and causes an increase in jitter. Such a problem of the intersymbol interaction is unavoidable in a conventional optical system, and seriously disturbs an increase in density. Returned light from the magneto-optical disc 5 is collimated via an objective lens 4, and is guided toward a photodetector 7 by a condenser lens 6. The aperture 38 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, and masks marginal rays in the track direction, so that these rays do not reach the photodetector 7. B in Fig. 62 represents the width of a light beam of the most marginal portion of the light-receiving system, and the width B is determined by the aperture 38. The width B represents the NA of the light-receiving system of the objective lens. Fig. 62 illustrates rays which pass various height positions of the pupil diameter, and of these rays, hatched rays which are masked by the aperture 38 form a side lobe in the light spot on the disc under the influence of the aperture 4' of the objective lens 4. Therefore, most of information components reproduced from an adjacent mark on the single track are included in these marginal rays, and the intersymbol interaction can be reduced by masking these rays. The side lobe due to the influence of the aperture 4' and marginal rays in the pupil of the light-receiving system normally have a good correspondence therebetween although they do not have a strict one-to-one correspondence therebetween unlike that between the light spot on the disc and the light spot of the light-receiving system shown in Fig. 4. Similarly, Fig. 63 illustrates, beside the light-receiving system, a state wherein a side lobe due to the influence of the aperture 4' of the objective lens 4 is generated in the light spot 9. The a-a' section of the light spot corresponds to the radial direction. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 by the condenser lens 6. The aperture 38 has a dimension in the radial direction larger than the beam diameter A so as not to mask marginal rays in the radial direction. This is to guide modulated components of a tracking signal from the disc to the photodetector as much as possible since these components are spatially distributed in the radial direction in the pupil of the light-receiving system. Fig. 64 is a front view of the aperture 38. The dimension, in the track direction, of the aperture is B2 (B2 < A2), and the dimension, in the radial direction, of the aperture is C2 (C2 > A2). If the objective lens has a focal length fo = 3 mm and NA = 0.55, the beam diameter A2 = 3.3 mm. As will be described later, since B2 is selected to be about several mm, easy adjustment of the aperture is greatly improved as compared to the prior art in which a pinhole having a diameter of 15 µm is inserted in the focal plane of the light-receiving system, and the aperture is not easily influenced by a change in temperature or aging. Since the aperture 38 is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and this arrangement is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. Fig. 65 shows the experimental results of a jitter reduction effect obtained using combinations of two optical heads (optical heads 1 and 2) and four discs (discs 1 to 4) in the optical system according to the third embodiment of the present invention. Optical head 1 has a wavelength λ = 780 nm, and the NA = 0.55 of the objective lens, and optical head 2 has a wavelength λ = 680 nm, and the NA = 0.60 of the objective lens. Disc 1 has a track pitch p = 1.4 µm, and a shortest mark length = 0.75 µm of a carrier (reproduction signal), disc 2 has a track pitch p = 1.1 µm, and a shortest mark length = 0.64 µm of a carrier, disc 3 has a track pitch p = 1.6 µm, and a shortest mark length = 0.78 µm of a carrier, and disc 4 has a track pitch p = 0.8 µm, and a shortest mark length = 0.47 µm of a carrier. Experimental conditions include: mark edge recording, a 1-7 modulation method for symbols, and a linear velocity = 15 m/s. The tilt and substrate thickness error of each disc are negligible. Fig. 65 shows the calculation results for a combination of optical head 1 and disc 1 as the first example (mark □ in Fig. 65). Fig. 65 shows the calculation results for a combination of optical head 2 and disc 2 as the second example (mark  in Fig. 65). Fig. 65 shows the calculation results for a combination of optical head 1 and disc 2 as the third example (mark ○ in Fig. 65). Fig. 65 shows the calculation results for a combination of optical head 1 and disc 3 as the fourth example (mark Δ in Fig. 65). Fig. 65 shows the calculation results for a combination of optical head 2 and disc 4 as the fifth example (mark × in Fig. 65). Fig. 65 shows the calculation results for a combination of optical head 2 and disc 1 as the sixth example (mark ▪ in Fig. 65). In Fig. 65, the abscissa represents the ratio of the width B2, in the track direction, of the aperture 38 to the beam diameter A2, and the ordinate represents the jitter amount. For example, in the combination of the first example, the reduction effect is maximized when B2/A2 is set to be about 0.75, and is gradually reduced when B2/A2 is set to be 0.75 or less. Although information from an adjacent mark is masked by the aperture 38, since the carrier level is lowered as B2/A2 becomes smaller, the C/N ratio deteriorates and the jitter amount due to noise increases. As can be seen from the above description, an aperture ratio B2/A2 which minimizes the jitter amount varies depending on combinations of optical heads and discs. In order to examine the relationship between the jitter reduction effect and the combinations of the optical heads and discs when the aperture is used, Table 7 below summarizes the light spot diameter determined by the NA of the objective lens and the wavelength of the semiconductor laser of the optical head, and the shortest mark length of the disc. Assuming that the light spot diameter is symmetrical in the radial and track directions, Table 1 is quoted. (Spot Diameter and Shortest Mark Length) Shortest Mark Length (µm) d2/m Optical Head 1 λ = 780 nm NA = 0.55 Optical Head 2 λ = 680 nm NA = 0.60 0.47 2.11 0.64 1.97 1.55 0.75 1.68 1.32 0.78 1.62 Table 7 shows the ratio d2/m of the 1/e 2 diameter d of the light spot shown in Table 1 and the shortest mark length m of the disc. As can be understood from Fig. 65 and Table 7, when the coma and the spherical aberration are satisfactorily corrected, the aperture ratio (B2/A2) for effectively reducing the jitter amount caused by, e.g., the intersymbol interaction is an amount associated with the light spot diameter determined by the NA of the objective lens of the optical head and the wavelength of the semiconductor laser, and the shortest mark length of the disc. Therefore, it can be estimated from this fact that an optimal ratio of the aperture width in the track direction to the beam diameter of the return light includes these parameters. The aperture ratio (B2/A2) which can minimize the jitter amount in the respective calculation results will be examined below with reference to the combinations of optical heads and discs. Fig. 66 shows the examination results of the relationship between B2/A2 and d2/m. In Fig. 66, the abscissa represents d2/m, and the ordinate represents the aperture ratio (B2/A2) which can minimize the jitter amount. As can be seen from Fig. 66, the combinations of the first to sixth examples are distributed almost on a straight line. An approximation of the straight line is as follows: B2/A2 = 0.92 - 0.1•(d2/m) As is apparent from Fig. 65, an aperture ratio range with a relatively low jitter amount of about ±0.15, which range has the minimum jitter amount given by equation (33) as the center, exists. When the aperture ratio is selected within a range which satisfies the following relation, the jitter amount caused by, e.g, the intersymbol interaction can be minimized, and stable signal reproduction is allowed. 0.77 - 0.1•(d2/m) < B2/A2 < 1.07 - 0.1•(d2/m) More preferably, the aperture ratio range satisfies: 0.82 - 0.1•(d2/m) < B2/A2 < 1.02 - 0.1•(d2/m) When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, information reproduced from an adjacent mark on a single track can be masked, and the jitter amount caused by the intersymbol interaction can be reduced. Note that the optical system described above with reference to Figs. 39 and 40 does not suffer a decrease in amplitude of a tracking signal which poses a problem in the optical system shown in Fig. 15 since rays in the radial direction are not masked although a single detection system is used for detecting a magneto-optical signal and a tracking servo signal. In this embodiment, although some rays in the track direction are masked by the aperture 38, they do not largely influence a servo signal. Therefore, independently of whether a single detection signal is used or independent detection systems are used for detecting a magneto-optical signal and servo signals, the aperture ratio preferably satisfies inequality (34) or (35). Fig. 67 shows the experimental results of the jitter reduction effect obtained using the combinations of optical head 1 with disc 1 and optical head 2 with disc 2 in the optical system of the embodiment described above with reference to Figs. 34 and 35. The experimental conditions are the same as those in the 10th embodiment of the present invention. The disc tilts are 3.9 mrad. and 6.5 mrad. for the combination of optical head 1 and disc 1, and are 2.6 mrad. and 4.4 mrad. for the combination of optical head 2 and disc 2. Depending on the difference in NA of the objective lens and λ of the wavelength, optical head 2 generates a coma about 1.5 times that generated by optical head 1. From equation (11), in optical head 1, when the disc is tilted at 3.9 mrad., W31 = 0.188λ; when the disc is tilted at 6.5 mrad., W31 = 0.313λ. In optical head 2, when the disc is tilted at 2.6 mrad. and 4.4 mrad., the same comas as described above are generated. The abscissa represents the ratio of the width B2, in the track direction, of the aperture 38 to the beam diameter A2, and the ordinate represents the jitter amount. For example, in the combination of optical head 1 and disc 1, when the disc is not tilted (W31 = 0), the reduction effect is maximized when B2/A2 is set to be about 0.75. However, when a disc tilt = 6.5 mrad. is generated (W31 = 0.313λ), the reduction effect is maximized when B2/A2 is set to be about 0.71. Depending on the disc tilt, the aperture ratio B2/A2 which minimizes the jitter amounts gradually shifts toward a smaller value. Although not shown, in combinations of optical head 1 with discs 2 and 3, and optical head 2 with discs 1 and 4, the same disc tilts as described above were given, and the same experiments were performed. In order to examine the relationship between the jitter reduction effect when an aperture is used in an optical disk optical system which suffers a coma due to, e.g., the disc tilt, and the combinations of optical heads and discs, Table 8 summarizes the light spot diameter determined by the NA of the objective lens of the optical head and the wavelength of the semiconductor laser, and the shortest mark length of the disc. Assuming that the light spot diameter is symmetrical in the radial and track directions, Table 3 is quoted. (Spot Diameter and Track Pitch) Shortest Mark Length (µm) d2/m Optical Head 1 λ = 780 nm NA = 0.55 Optical Head 2 λ = 680 nm NA = 0.60 Disc Tilt (mrad.) 3.9 6.5 2.6 4.4 0.47 2.17 2.24 0.64 2.00 2.03 1.59 1.64 0.75 1.70 1.73 1.36 1.40 0.78 1.64 1.66 Table 8 shows the ratio d2/m of the 1/e 2 diameter d of the light spot shown in Table 3 and the shortest mark length m of the disc. As can be understood from Fig. 67 and Table 8, when a coma is generated due to, e.g., a disc tilt, the aperture ratio (B2/A2) for effectively reducing the jitter amount caused by, e.g., the intersymbol interaction is an amount associated with the light spot diameter determined by the NA of the objective lens of the optical head and the wavelength of the semiconductor laser, the shortest mark length of the disc, and the coma coefficient W31. Therefore, it can be estimated from this fact that an optimal ratio of the aperture width in the track direction to the beam diameter of the return light includes these parameters. The aperture ratio (B2/A2) which can minimize the jitter amount in the respective calculation results will be examined below with reference to the combinations of optical heads and discs. Fig. 68 shows the examination results of the relationship between B2/A2 and d2/m. In Fig. 68, the abscissa represents d2/m, and the ordinate represents the aperture ratio (B2/A2) which can minimize the jitter amount. As can be seen from Fig. 68, the combinations of the first to sixth examples are distributed almost on a straight line. An approximation of the straight line is as follows: When W31 = 0 (when the disc is not tilted), from equation (31), we have: B2/A2 = 0.92 - 0.1•(d2/m) When W31 = 0.188λ, we have: B2/A2 = 0.90 - 0.1•(d2/m) When W31 = 0.313λ, we have: B2/A2 = 0.88 - 0.1•(d2/m) As can be seen from these equations, when the coma increases, B2/A2 must be decreased in proportion to the coma coefficient so as to decrease the jitter amount caused by, e.g., the intersymbol interaction. From equations (33), (36), and (37), when W31 is introduced in these equations, we have: B2/A2 = 0.92 - 0.1•(d2/m) - 0.12•W31 Also, as is apparent from Fig. 67, an aperture ratio range with a relatively low jitter amount of about ±0.15, which range has the minimum jitter amount given by equation (33) as the center, exists. When the aperture ratio is selected within a range which satisfies the following relation, the jitter amount caused by, e.g, the intersymbol interaction can be minimized, and stable signal reproduction is allowed. 0.77 - 0.1•(d2/m) - 0.12•W31 < B2/A2 < 1.07 - 0.1•(d2/m) - 0.12•W31 More preferably, the aperture ratio range satisfies: 0.82 - 0.1•(d2/m) - 0.12•W31 < B2/A2 < 1.02 - 0.1•(d2/m) - 0.12•W31 For W31 ≠ 0, 0 < B2/A2 < 1 When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, information reproduced from the adjacent mark is masked, and the jitter amount due to, e.g., the intersymbol interaction can be reduced. The arrangement according to the fourth embodiment of the present invention will be described below with reference to Figs. 69 and 70. Fig. 69 is a sectional view, in the track direction, of a magneto-optical disc 5 in an optical system of the present invention, and Fig. 70 is a sectional view, in the radial direction, of the disc 5. Figs. 69 and 70 particularly illustrate only the light-receiving system for explaining the principle of the present invention. Fig. 69 illustrates, beside the light-receiving system, a state wherein a side lobe due to a spherical aberration is generated in a light spot 9. The spherical aberration is generated due to a manufacturing error of an objective lens and a disc substrate thickness error, and has a side lobe which is symmetrical about the axis of rotation. Although the light spot shown in Fig. 62 has a side lobe which is symmetrical about the axis of rotation, when a spherical aberration is generated, a side lobe having a higher peak intensity is observed. The light spot is illustrated in a state observed in the direction of rays, and its b-b' section corresponds to the track direction. When information of a mark on a given track is reproduced in this state, the side lobe due to the spherical aberration reproduces information of an adjacent mark on the single track, and the reproduced information generates a distortion or shift in a reproduction signal to be originally reproduced. This is called an intersymbol interaction, and causes an increase in jitter. When the NA of the objective lens is to be increased, the allowable manufacturing error of the objective lens and the disc substrate thickness error must be reduced, and a problem of the intersymbol interaction from an adjacent mark due to the spherical aberration seriously disturbs an increase in density. Returned light from the magneto-optical disc 5 is collimated via an objective lens 4, and is guided toward a photodetector 7 by a condenser lens 6. An aperture 38 is arranged between the objective lens 4 and the condenser lens 6, i.e., in the vicinity of the pupil of the light-receiving system, and masks marginal rays in the track direction, so that these rays do not reach the photodetector 7. Fig. 69 illustrates rays which pass various height positions of the pupil diameter, and as can be seen from Fig. 69, of these rays, hatched rays which are masked by the aperture 38 mainly form a side lobe in the light spot on the disc. Therefore, most of information components reproduced from an adjacent mark on a single track are included in these marginal rays, and the intersymbol interaction can be reduced by masking these rays. The side lobe due to the spherical aberration and marginal rays in the pupil of the light-receiving system normally have a good correspondence therebetween although they do not have a strict one-to-one correspondence therebetween unlike that between the light spot on the disc and the light spot of the light-receiving system shown in Fig. 4. In Fig. 70, an arrow 13 and the a-a' section of the light spot correspond to the radial direction. Returned light from the magneto-optical disc 5 is collimated via the objective lens 4, and is guided toward the photodetector 7 by the condenser lens 6. The aperture 38 has a dimension in the radial direction larger than the beam diameter A so as not to mask marginal rays in the radial direction. This is to guide modulated components of a tracking signal from the disc to the photodetector as much as possible since these components are spatially distributed in the radial direction in the pupil of the light-receiving system. Fig. 71 shows the experimental results of the jitter reduction effect using combinations of optical head 1 with disc 2 and optical head 2 with disc 2 in the optical system according to the fourth embodiment of the present invention. The experimental conditions are the same as those in the third embodiment of the present invention. The substrate thickness errors of the disc are ±50 µm, ±75 µm, and ±100 µm for the combination of optical head 1 and disc 1, and are ±31 µm, ±46 µm, and ±62 µm for the combination of optical head 2 and disc 2. Depending on the differences in NA of the objective lens and in wavelength, optical head 2 generates a spherical aberration about 1.6 times that generated by optical head 1. In equation (19), when optical head 1 suffers substrate thickness errors of ±50 µm, ±75 µm, and ±100 µm, W40 = 0.28λ, 0.41λ, and 0.55λ, respectively. On the other hand, when optical head 2 suffers substrate thickness errors of ±31 µm, ±46 µm, and ±62 µm, equivalent spherical aberrations are generated. The abscissa represents the ratio of the width B2, in the track direction, of the aperture 38 to the beam diameter A2, and the ordinate represents the jitter amount. For example, in the combination of optical head 1 and disc 1, when the disc suffers no substrate thickness error (W40 = 0), the reduction effect is maximized when B2/A2 is set to be about 0.75. However, when a disc substrate error = ±100 µm is generated (W40 = 0.55λ), the reduction effect is maximized when B2/A2 is set to be about 0.715. Depending on the substrate thickness error, the aperture ratio B2/A2 which minimizes the jitter amounts gradually shifts toward a smaller value. Although not shown, in combinations of optical head 1 with discs 2 and 3, and optical head 2 with discs 1 and 4, the same substrate thickness errors as described above were given, and the same experiments were performed. In order to examine the relationship between the jitter reduction effect when an aperture is used in an optical disk optical system which suffers a spherical aberration due to, e.g., the substrate thickness error, and the combinations of optical heads and discs, Table 9 summarizes the light spot diameter determined by the NA of the objective lens of the optical head and the wavelength of the semiconductor laser, and the shortest mark length of the disc. Assuming that the light spot diameter is symmetrical in the radial and track directions, Table 5 is quoted. (Spot Diameter and Track Pitch) Shortest Mark Length (µm) d2/m optical Head 1 λ = 780 nm NA = 0.55 Optical Head 2 λ = 680 nm NA = 0.60 Substrate Thickness Error (µm) 50 75 100 31 46 62 0.47 2.11 2.11 2.13 0.64 1.97 1.97 1.98 1.55 1.55 1.56 0.75 1.68 1.68 1.69 1.32 1.32 1.33 0.78 1.62 1.62 1.63 Table 9 shows the ratio d2/m of the 1/e 2 diameter d of the light spot shown in Table 5 and the shortest mark length m of the disc. As can be understood from Fig. 71 and Table 9, when a spherical aberration is caused by a substrate thickness error, the aperture ratio (B2/A2) for effectively reducing the jitter amount caused by, e.g., the intersymbol interaction is an amount associated with the light spot diameter determined by the NA of the objective lens of the optical head and the wavelength of the semiconductor laser, the shortest mark length of the disc, and the spherical aberration coefficient W40. Therefore, it can be estimated from this fact that an optimal ratio of the opening width in the track direction to the beam diameter of the return light includes these parameters. The aperture ratio (B2/A2) which can minimize the jitter amount in the respective calculation results will be examined below with reference to the combinations of optical heads and discs. Fig. 72 shows the examination results of the relationship between B2/A2 and d2/m. In Fig. 72, the abscissa represents d2/m, and the ordinate represents the aperture ratio (B2/A2) which can minimize the jitter amount. As can be seen from Fig. 72, the combinations of the first to sixth examples are distributed almost on a straight line. An approximation of the straight line is as follows: When W40 = 0 (when the disc suffers no substrate thickness error), from equation (33), we have: B2/A2 = 0.92 - 0.1•(d2/m) When W40 = 0.28λ, we have: B2/A2 = 0.91 - 0.1•(d2/m) When W40 = 0.41λ, we have: B2/A2 = 0.90 - 0.1•(d2/m) When W40 = 0.55λ, we have: B2/A2 = 0.88 - 0.1•(d2/m) As can be seen from these equations, when the spherical aberration increases, B2/A2 must be decreased in proportion to a square of the spherical aberration coefficient so as to decrease the jitter amount caused by, e.g., the intersymbol interaction. From equations (33), and (41) to (43), when W40 is introduced in these equations, we have: B2/A2 = 0.92 - 0.1•(d2/m) - 0.12•W40 2 Also, as is apparent from Fig. 71, an aperture ratio range with a relatively low jitter amount of about ±0.15, which range has the minimum jitter amount given by equation (44) as the center, exists. When the aperture ratio is selected within a range which satisfies the following relation, the jitter amount caused by, e.g, the intersymbol interaction can be minimized, and stable signal reproduction is allowed. 0.77 - 0.1•(d2/m) - 0.12•W40 2 < B2/A2 < 1.07 - 0.1•(d2/m) - 0.12•W40 2 More preferably, the aperture ratio range satisfies: 0.82 - 0.1•(d2/m) - 0.12•W40 2 < B2/A2 < 1.02 - 0.1•(d2/m) - 0.12•W40 2 For W40 ≠ 0, 0 < B2/A2 < 1 When such an aperture is inserted in the vicinity of the pupil of the light-receiving system, information reproduced from an adjacent mark is masked, and the jitter amount due to, e.g., the intersymbol interaction can be reduced. Contributions of the coma and the spherical aberration to the light spot are independent, and an increase in jitter amount due to the intersymbol interaction is independently caused by the coma and the spherical aberration. Therefore, it is confirmed based on simulation and experimental results that the total jitter amount upon simultaneous generation of the two aberrations corresponds to a square mean of independently generated crosstalk amounts. Thus, setting of an optimal aperture ratio corresponding to d2/m upon simultaneous generation of the two aberrations will be examined below. From equations (38) and (44), since the optimal aperture ratio B2/A2 corresponds to a square mean of the contribution amounts of a coma and a spherical aberration, it can be expressed by: B2/A2 = 0.92 - 0.1•(d2/m) - √{(0.12•W31) 2 + (0.12•W40 2 ) 2 } When a disc tilt and a disc substrate thickness error are simultaneously generated, from inequalities (39) and (45), B2/A2 preferably satisfies: 0.77 - 0.1•(d2/m) - √{(0.12•W31) 2 + (0.12•W40 2 ) 2 } < B2/A2 < 1.07 - 0.1•(d2/m) - √{(0.12•W31) 2 + (0.12•W40 2 ) 2 } More preferably, from inequalities (40) and (46), B2/A2 satisfies: 0.82 - 0.1•(d2/m) - √{(0.12•W31) 2 + (0.12•W40 2 ) 2 } < B2/A2 < 1.02 - 0.1•(d2/m) - √{(0.12•W31) 2 + (0.12•W40 2 ) 2 } For W31 ≠ 0, W40 ≠ 0, 0 < B2/A2 < 1 For example, in the combination of the first example (d2/m = 1.68), when a disc tilt = 4 mrad. (W31 = 0.2λ or equivalent) and the disc substrate thickness error = 80 µm (W40 = 0.44λ or equivalent) are simultaneously generated, the aperture 12 preferably has B2/A2 = 0.57 to 0.87. More preferably, the aperture has B2/A2 = 0.62 to 0.82. As described above, when marginal rays in the track direction of the light-receiving system are masked, information reproduced from an adjacent mark can be masked, and the jitter amount caused by the intersymbol interaction can be reduced. As in the above embodiments in which crosstalk components from an adjacent track are reduced by masking marginal rays in the radial direction, means for masking marginal rays in the track direction may be arranged in the vicinity of the pupil of the light-receiving system or may be arranged in the far field region sufficiently separated from the focal point of the light-receiving system. An optimal ratio of the aperture width in the track direction to the beam diameter of a returned light beam (aperture ratio B2/A2) can be set as follows. (1) When coma and spherical aberration are satisfactorily corrected: an aperture ratio determined by the ratio d/m of the light spot diameter d of the optical head in the track direction to the shortest mark length recorded on the disc can be used. (2) When coma is dominant: an aperture ratio determined by the ratio d/m of the light spot diameter d of the optical head in the track direction to the shortest mark length recorded on the disc, and the wave aberration coefficient W31 of the coma can be used. (3) When spherical aberration is dominant: an aperture ratio determined by the ratio d/m of the light spot diameter d of the optical head in the track direction to the shortest mark length recorded on the disc, and the wave aberration coefficient W40 of the spherical aberration can be used. (4) When both coma and spherical aberration are simultaneously generated: an aperture ratio determined by the ratio d/m of the light spot diameter d of the optical head in the track direction to the shortest mark length recorded on the disc, the wave aberration coefficient W31 of the coma, and the wave aberration coefficient W40 of the spherical aberration can be used. Therefore, even when a disc suffers from a tilt or a substrate thickness error, information reproduced from an adjacent mark can be masked, and the jitter amount due to the intersymbol interaction can be reduced. The aperture 38 may have either a rectangular shape or an elliptic shape. Marginal rays in the track direction may be prevented from being received by modifying the shape of the photodetector. These arrangements may be applied to either an optical system which includes a single detection system for a magneto-optical signal and servo signals, or an optical system which includes independent detection systems therefor. The reduction of jitter components caused by a coma due to a disc tilt or a spherical aberration due to a substrate thickness error has been particularly exemplified. The present invention is also effective when an astigmatism or a defocus is generated. When the present invention is used, easy adjustment of the aperture or photodetector is greatly improved as compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and the arrangement of the present invention is not easily influenced by a change in temperature or aging. Since the aperture is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and the arrangement of the present invention is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. As has been described above, when an aperture for masking marginal rays, in the radial direction, of returned light from a disc is arranged in a region sufficiently separated from the focal plane of the optical head light-receiving system, i.e., in the vicinity of the pupil of the light-receiving system, the influence of crosstalk components from an adjacent track can be effectively reduced. The same effect as described above can be expected by masking marginal rays, in the radial direction, of returned light by modifying the shape of the photodetector. As has been described above, when an aperture for masking marginal rays, in the track direction, of returned light from a disc is arranged in a region sufficiently separated from the focal plane of the optical head light-receiving system, i.e., in the vicinity of the pupil of the light-receiving system, information reproduced from an adjacent mark can be masked, and jitter components caused by the intersymbol interaction can be reduced. The same effect as described above can be expected by masking marginal rays, in the track direction, of returned light by modifying the shape of the photodetector. The present invention can be widely applied to either an optical system which includes a single detection system for a magneto-optical signal and servo signals, or an optical system which includes independent detection systems therefor. When the present invention is used, easy adjustment of the aperture or photodetector is greatly improved as compared to the prior art in which a pinhole is inserted in the focal plane of the light-receiving system, and the arrangement of the present invention is not easily influenced by a change in temperature or aging. Since the aperture is inserted in the light-receiving system, a considerable decrease in light utilization efficiency due to the insertion of an annular aperture can be prevented unlike in one prior art, and the arrangement of the present invention is suitable for, e.g., a magneto-optical disc recording/reproduction apparatus which requires high power upon recording/erasing of information. According to the present invention, crosstalk components from an adjacent track and intersymbol interaction factors from an adjacent mark in the track direction due to a disc tilt can be effectively removed, and the allowable amount of the disc tilt is lessened, thus reducing cost of the disc. Also, crosstalk components from an adjacent track and intersymbol interaction factors from an adjacent mark in the track direction due to a disc substrate thickness error can be effectively removed, and the allowable amount of the substrate thickness error is lessened, thus reducing cost of the disc. Since crosstalk components from an adjacent track or intersymbol interaction factors from an adjacent mark in the track direction, which are included in a side lobe portion can be effectively removed by inserting a simple aperture or modifying the shape of the photodetector, the optical head can be rendered compact, and requirements as to the tilt adjustment precision, performance, and the like of the objective lens are rendered less strict, thus reducing cost.

Claims: An optical recording/reproduction apparatus, comprising: an irradiation optical system (1,2,3) for irradiating a light beam from a light source onto a predetermined track of an optical recording medium (5) having a plurality of neighboring tracks as a fine light spot (9) so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system (7;7-1,7-2) for detecting a returned light beam from the optical recording medium; and mask means (38), arranged in a far field region sufficiently separated from a focal plane of said detection optical system (7;7-1,7-2) in an optical path of said detection optical system (7;7-1,7-2), for masking marginal rays, in a track direction, of the returned light beam, the apparatus being characterised in that said mask means (38) satisfies the following relation: 0.77 - 0.1•(d2/m) < B2/A2 < 1.07 - 0.1• (d2/m) for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means (38) for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot (9) on the optical recording medium (5) in the track direction, and m is the shortest mark length on the optical recording medium (5), whereby information reproduced by the light spot (9) from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot (9) is reduced. An optical recording/reproduction apparatus comprising: an irradiation optical system (1,2,3) for irradiating a light beam from a light source onto a predetermined track of an optical recording medium (5) having a plurality of neighboring tracks as a fine light spot (9) so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system (7;7-1,7-2) for detecting a returned light beam from the optical recording medium; and mask means (39), arranged in a far field region sufficiently separated from a focal plane of said detection optical system (7;7-1,7-2) in an optical path of said detection optical system, for masking marginal rays, in a track direction, of the returned light beam, wherein a dominant optical aberration of the light spot is a coma, the apparatus being characterised in that said mask means satisfies the following relation: 0.77 - 0.1•(d2/m) - 0.12•W31 < B2/A2 < 1.07 - 0.1• (d2/m) - 0.12•W31 for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means (39) for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot on the optical recording medium (5) in the track direction, m is the shortest mark length on the optical recording medium (5), and W31 is the wave aberration coefficient of the coma whereby information reproduced by a side lobe, generated by the coma, of the light spot (9) from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot (9) is reduced. An optical recording/reproduction apparatus comprising: an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium (5) having a plurality of neighboring tracks as a fine light spot (9) so as to perform recording/reproduction of information or reproduction of information by time-serially scanning a plurality of marks located on the predetermined track; a detection optical system (7) for detecting a returned light beam from the optical recording medium; and mask means (38), arranged in a far field region sufficiently separated from a focal plane of said detection optical system (7) in an optical path of said detection optical system (7), for masking marginal rays, in the track direction, of the returned light beam, wherein a dominant optical aberration of the light spot is a spherical aberration; the apparatus being characterised in that said mask means (38) satisfies the following relation: 0.77 - 0.1•(d2/m) - 0.12•W40 2 <B2/A2 < 1.07 - 0.1• (d2/m) - 0.12•W40 2 for 0 < B2/A2 < 1 where A2 is the beam diameter of the returned light beam in the track direction, B2 is the aperture width of said mask means (38) for masking the returned light beam in the track direction, d2 is the 1/e 2 diameter of the light spot on the optical recording medium in the track direction, m is the shortest mark length on the optical recording medium (5), and W40 is the wave aberration coefficient of the spherical aberration, whereby information reproduced by a side lobe, generated by the spherical aberration, of the light spot from a mark adjacent to a predetermined mark on the predetermined track upon reproduction of the predetermined mark located on the predetermined track by the light spot is reduced. An apparatus according to claim 1, 2 or 3, wherein said mask means (38) comprises an aperture smaller than a beam diameter, in the track direction, of the returned light beam. An apparatus according to claim 1, 2 or 3, wherein said mask means (38) comprises a photodetector which has a light-receiving portion smaller than a beam diameter, in the track direction, of the returned light beam. An apparatus according to claim 1, 2 or 3 wherein said mask means (38) is arranged in the vicinity of a pupil of said detection optical system. An apparatus according to claim 1, 2 or 3 wherein said mask means (38) is arranged in an optical path in which the returned light beam from the optical recording medium (5) is collimated.

Inventors: KOYAMA OSAMU, YAMAWAKI TAKESHI, KOYAMA, OSAMU, YAMAWAKI, TAKESHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: G11B7
IPC codes: G11B7, G11B11